Unstable water flow in a layered soil: II. Effects of an unstable water-repellent layer

Water repellent soils are found throughout the world and can exhibit signif icantly different water now characteristics as compared to a wettable soil. The purpose of the study was to determine the significance of the stabilit y of the water repellency on the development of unstable water flow below a water repellent layer. Unstable water-repellent soil refers to a soil whos e degree of repellency changes with time after contact with water, Experime nts were conducted in a specially built rectangular chamber where wetting f ront patterns could be observed through a Plexiglas sheet. The experiments were done on water repellent sand layers that were treated to create water drop penetration time (WDPT) values of 1, 10, and 150 min. The WDPT of the layer and the ratio (h(o) + L)/h(p) were important in the development of fi ngers, where h(o) is the depth of ponded water at the soil surface, L is th e depth of the water repellent layer and h(o), is the water entry pressure head of the water repellent layer. For low WDPT (1 min) no fingers formed, As the WDPT increased, the tendency for finger:formation also increased. Th e medium WDPT (10 min) layer caused finger formation, however, the fingers broadened and converged after continued now and an almost uniform wetting f ront eventually developed, The combination of a high WDPT (150 min) and (h( o) + L)/h(p) < 1 produced the most dramatic and persistent fingering. The f inger development across the layer and the flux through the layer was found to be a function of time. Water repellency at the soil surface has the gre atest impact on infiltration because water depth may not be sufficient to o vercome the water entry pressure and runoff would decrease the time of expo sure to water to overcome unstable water repellency.